Update 'this' extension receiver when there's a smart cast

If 'this' (implicit or explicit) was used as an extension receiver,
and the corresponding call required a smart-cast,
this information was effectively lost in "old" resolution & inference,
but is required by "old" JVM BE to generate proper CHECKCASTs.

compiler/frontend/src/org/jetbrains/kotlin/resolve/calls/CandidateResolver.kt

@@ -566,6 +566,9 @@ class CandidateResolver(
                 if (isDispatchReceiver) {
                     candidateCall.setSmartCastDispatchReceiverType(smartCastResult.resultType)
                 }
+                else {
+                    candidateCall.updateExtensionReceiverWithSmartCastIfNeeded(smartCastResult.resultType)
+                }
                 if (!smartCastResult.isCorrect) {
                     // Error about unstable smart cast reported within checkAndRecordPossibleCast
                     return UNSTABLE_SMARTCAST_FOR_RECEIVER_ERROR

compiler/frontend/src/org/jetbrains/kotlin/resolve/calls/model/MutableResolvedCall.java

@@ -67,4 +67,6 @@ public interface MutableResolvedCall<D extends CallableDescriptor> extends Resol
     boolean hasInferredReturnType();
 
     void setSmartCastDispatchReceiverType(@NotNull KotlinType smartCastDispatchReceiverType);
+
+    void updateExtensionReceiverWithSmartCastIfNeeded(@NotNull KotlinType smartCastExtensionReceiverType);
 }

compiler/frontend/src/org/jetbrains/kotlin/resolve/calls/model/ResolvedCallImpl.java

@@ -33,6 +33,8 @@ import org.jetbrains.kotlin.resolve.calls.results.ResolutionStatus;
 import org.jetbrains.kotlin.resolve.calls.tasks.ExplicitReceiverKind;
 import org.jetbrains.kotlin.resolve.calls.tasks.ResolutionCandidate;
 import org.jetbrains.kotlin.resolve.calls.tasks.TracingStrategy;
+import org.jetbrains.kotlin.resolve.scopes.receivers.CastImplicitClassReceiver;
+import org.jetbrains.kotlin.resolve.scopes.receivers.ImplicitClassReceiver;
 import org.jetbrains.kotlin.resolve.scopes.receivers.ReceiverValue;
 import org.jetbrains.kotlin.types.KotlinType;
 import org.jetbrains.kotlin.types.TypeProjection;
@@ -60,7 +62,7 @@ public class ResolvedCallImpl<D extends CallableDescriptor> implements MutableRe
     private final D candidateDescriptor;
     private D resultingDescriptor; // Probably substituted
     private final ReceiverValue dispatchReceiver; // receiver object of a method
-    private final ReceiverValue extensionReceiver; // receiver of an extension function
+    private ReceiverValue extensionReceiver; // receiver of an extension function
     private final ExplicitReceiverKind explicitReceiverKind;
     private final TypeSubstitutor knownTypeParametersSubstitutor;
 
@@ -378,4 +380,14 @@ public class ResolvedCallImpl<D extends CallableDescriptor> implements MutableRe
     public KotlinType getSmartCastDispatchReceiverType() {
         return smartCastDispatchReceiverType;
     }
+
+    @Override
+    public void updateExtensionReceiverWithSmartCastIfNeeded(@NotNull KotlinType smartCastExtensionReceiverType) {
+        if (extensionReceiver instanceof ImplicitClassReceiver) {
+            extensionReceiver = new CastImplicitClassReceiver(
+                    ((ImplicitClassReceiver) extensionReceiver).getClassDescriptor(),
+                    smartCastExtensionReceiverType
+            );
+        }
+    }
 }